1. Field of the Invention
The present invention relates to a lens system, and particularly relates to a compact lens system with low cost and high performance for use in a compact photographic device such as a camera phone and a compact digital camera.
2. Description of Prior Art
Portable photographic devices, such as camera phones, compact digital cameras and personal digital assistances (PDAs), generally require its lens to be compact and low cost while maintaining a high level of optical performance. Due to the limited space, photographing lenses for the portable photographic devices have dimensions much smaller than those for common photographic cameras and video cameras. Accordingly, camera phones and compact digital cameras generally employ fixed focal length lenses.
As there is a large selection of glass available with different indices of refraction and dispersion properties, glass is generally used for photographing lenses. Glass lenses are manufactured by a grinding and polishing process, which is more expensive and hard to duplicate in mass manufacturing compared to plastic lenses. This process also makes it difficult and expensive to fabricate a lens with any other shape than spherical or planar. Further, spherical surface lenses have difficulty in correcting distortion in ultra-wide angle lenses or coma in large-aperture lenses brought about by spherical aberration. Accordingly, to address this problem, an aspheric lens is used. The aspheric lens may be a plastic lens formed by plastic molding or a glass lens formed by glass molding.
In the current market, most lens systems for compact photographic devices such as camera phones are generally composed of two plastic lens elements, or of one glass lens element and two plastic lens elements (referring to U.S. Pat. No. 6,441,971). The two plastic lens elements in those conventional lens systems are generally configured as positive lens elements that are asymmetrically arranged. This renders the lens system more sensitive to temperature variations and requires a high level of precision, which limits its extensive application. Also, aberrations cannot be effectively corrected, which significantly degrades the optical performance of the lens system. In addition, the two plastic lens elements are generally made of the same plastic material or of different plastic materials with similar refractive indices, which makes the correction of field curvature difficult. Further, the aspheric lens element employed in those conventional lens systems generally has a steep surface topography with varying radius of curvature, which makes the manufacture of the aspheric lens element difficult.
U.S. Pat. No. 6,031,670 discloses a lens system composed of two plastic lens elements, one of which is a negative meniscus lens element on the object side and the other is a positive biconvex lens element on the image side. Both of the two plastic lens elements are formed as aspheric lenses for aberration correction. However, the two aspheric lens elements are asymmetrically arranged. As a result, distortions of the optical system cannot be eliminated completely. In addition, the two plastic lens elements are made of the same material having the same refractive indices. Restriction to only one lens material makes it difficult to reduce the Petzval sum and thus image field curvature. The Petzval sum is the sum when the inverse of the product of the index of refraction and focal length of each lens element is added to the total number of lens elements making up the lens system.
U.S. Pat. No. 4,212,517 discloses a lens system of a modified Gauss type having the aperture stop between lens elements. This symmetry design of the lens elements allows well-balanced correction of all types of aberrations. However, this lens system is composed of six lenses. Consequently, the overall length of this lens system is increased, which deviates from the current trend for compactness. Various symmetry arrangements of lens elements are also disclosed in U.S. Pat. Nos. 4,364,643; 4,390,252; 4,396,255; 4,426,137; 4,443,070 and 4,448,497. However, these lens systems as disclosed also have the same problem of a large number of lens elements, and thus are not suitable for use in compact photographic devices.
Accordingly, a compact lens system for use in a compact photographic device is desired to overcome the above disadvantages present in the prior art.